Recently developed techniques allow direct control of the membrane voltage of cardiac muscle cells. These voltage-clamp methods are very useful in examining the coupling between excitation and contraction in the heart, since it is possible to measure membrane currents and tension under conditions such that membran voltage is under the control of the investigator. I propose to use the two-microelectrode technique of voltage-clamping the sheep cardiac Purkinje fiber to examine detail how cardiac contraction depends on membran voltage, ionic currents, and time. The principal goal of the project is to examine the behavior of the slow inward current (Is) and its relaton to contraction. Specific areas of investigation include: attempts to separate clearly the slow inward current from the chloride current from the chloride current (I Cl), which obscures Is at voltages positive to -20 mV; analysis of the Kinetics of Is and ICl; comparisons of the kinetics of Is to those of contraction; and experiments designed to examine the factors governing the rate of recovery of the contractile system in heart. The data should provide information about frequency-dependent contractile phenomena in the heart, such as the staircase of treppe phenomenon, the relation between stimulation frequency and strength of contraction, and the effects of paired-pacing. More should be learned about the regulation of myocardial contractility and excitation-contraction coupling in the heart. The experiments on the slow inward current may provide insight into the mechanisms responsible for some types of arrhythmias. Separation of Is and 1Cl would also be useful in analyzing the ionic basis of the cardiac action potential.